Taking a Mercurial Approach

By MICHAEL D. LEMONICK

The telescope sits on the lawn outside Vachon Pavilion at Quebec's Laval University, its gently concave 40-in.-diameter mirror pointing at the sky. Concentrating and reflecting faint starlight into a camera mounted above it, the gleaming face of the mirror seems devoid of the slightest imperfection; it is so smooth, in fact, that it looks solid.

But that is an illusion. The mirror is really a pool of liquid mercury in a shallow wood container. A touch would send ripples racing across its surface, and it must always aim straight up to retain its curvature. As the container is slowly rotated on a turntable; making one revolution every six seconds, the mercury rises gently toward the edges and dips in the middle, the way coffee does when it is stirred in a cup. In perfect deference to the laws of physics, the metal's highly reflective surface takes the form of a parabola, the shape of solid mirrors used in conventional telescopes to focus starlight into a sharp image. Says Ermanno Borra, the Laval astrophysicist who built it: "It's a wonderfully simple arrangement."

That arrangement may help astronomy break through a size barrier that it reached in 1948, when technicians completed work on the famous 200-in. glass mirror for the Hale Telescope at Mount Palomar; beyond that size, glass mirrors tend to sag and distort un-acceptably, affected both by their own weight and by changes in temperature. The only larger mirror in the world, a 236-in. monolith atop Mount Semirodriki in the Soviet Union, is apparently hopelessly flawed and has done little significant work since being completed in 1974. One solution to the size problem is to make several smaller mirrors work together, simulating a single large one. The computer-synchronized Multiple-Mirror Telescope atop Mount Hopkins in Arizona, for example, has six mirrors and the light-gathering power of a 176-in. instrument. But Borra claims that his mercurial approach will make possible virtually flawless single mirrors at least five times as big as the one at Palomar and can do it simply and inexpensively.

Borra emphasizes that the concept of a liquid-mirror telescope is not new; he thinks the idea may have occurred to Isaac Newton, who knew about the behavior of spinning fluids and built one of the first reflecting telescopes. Borra knows that Robert Wood of Johns Hopkins University built a primitive model in 1908. "I am not the inventor," says Borra. "But I am the first to make it work and the first to know what to do with it."

Wood's problem was that the motors of his day could not turn at a constant enough rate of speed. As a result, the curvature of his mirror kept changing. Also he was unable to avoid vibrations, which set up ripples in the metallic pool. Wood was aware of another shortcoming of his telescope: because it always had to face straight up, it could not be swung around to point at interesting stars and galaxies or to take time-exposure photographs by following the celestial objects across the sky as the earth rotated.

It was apparent to Borra that modern technology, in the form of a precise synchronous motor to turn his mirror and nearly frictionless, vibration-free air bearings to support it, could solve the mechanical problems. And he realized that despite the mirror's pointing limit, there would be a legitimate use for it. "If your study is cosmology, the origins and nature of the universe, it doesn't matter where you look," he says. "There are data to be found anywhere you aim a telescope."

Although pointing only straight up, the telescope at any given moment covers a region of sky twice the width of the full moon and, as the earth turns, scans a narrow but long band of the heavens. With many liquid telescopes set up at different latitudes, says Borra, astronomers could map the locations of the faintest, farthest galaxies, find out how they are distributed in space and thereby learn a great deal about the structure and evolution of the universe. Or they could take a brief exposure of a particular galaxy each night, store the image digitally, then add to it with exposures on following nights until enough data had accumulated to reveal previously unseen details.

When Borra began building a working model in 1982, another advantage of the concept soon became apparent. The 40-in. model cost him less than $7,500 (U.S.), and he has a grant of $22,500 from the Natural Sciences and Engineering Research Council of Canada for a 60-in. version now under construction. Says Borra: "I think that we could build a liquid mirror telescope as large as 30 meters [1,181 in.] for $7.5 million." By contrast, the 400-in., 36-segment mirror alone for the Keck Telescope, to be erected in Hawaii, is expected to cost $25 million.

Borra reports that other astronomers have taken notice of his venture: "They've said, 'It's an interesting concept--now show us." Although Roger Angel, head of the University of Arizona's Steward Observatory Mirror Laboratory, has not given up on larger glass mirrors (his team has designed an inexpensive 312-in. mirror with a unique honeycomb structure to keep it light), he calls the liquid-mercury project "very challenging. If it can be done at 30 meters, it be comes unique in its grasp of light. There are problems in astrophysics that would lend themselves to this kind of telescope."

Many challenges still await Borra. "It is simply more difficult to build things bigger," he says. "We would have to build bigger turntables and mount them on bigger bearings. We would have to use bigger motors to turn them, and we would encounter problems of weight." Surface ripples present another problem, but a thin coating of oil suppresses them. The layer of tarnish and dirt that slowly dulls the mercury's reflective surface is easily skimmed off. Says Borra: "We've proved the hardest thing: you can make an astronomical-quality mirror out of liquid mercury. These mirrors won't give us answers to the ultimate questions about the origins of the uni verse, but they should help us understand enough so that we can ask these questions."

With reporting by Reported by Peter Stoler/Quebec City